I found wiki not that clear for installation of commercial certificates. So i am listing step by step process which will be good for ppl. who are facing isues with installing commercial certificates: -

STEPS TO INSTALL COMMERCIAL TRIAL CERTIFICATE

1. GO IN /opt/zimbra/ssl/zimbra/commercial directory AND SEE IF commercial.key has permission set to 740.
2. IF NOT CHANGE PERMISSIONS
3. With CSR get certificate from verisign
4. create a temp dir say /root/certs
5. save certificate file in /root/cert as commercial.crt
6. chmod TO 700 commercial.crt
7. In mail which you recieved from verisign also look for root and intermediate certificate links.
8. SAVE THEM IN /ROOT/CERTS DIRECTORY AS ROOT.CA AND INTERMEDIATE.CA
9. RUN cat root.ca intermediate.ca >> commercial_ca.crt
10. RUN chmod 770 commercial_ca.crt
11. VERIFY USING THIS COMMAND
/opt/zimbra/bin/zmcertmgr verifycrt comm /path/to/privatekey /path/to/commercial.crt /path/to/commercial_ca.cr